Methods and power controllers for primary side control

ABSTRACT

Power controllers and related primary-side control methods are disclosed. A disclosed power controller has a comparator and an ON-triggering controller. The comparator compares a feedback voltage with an over-shot reference voltage. Based on an inductance-coupling effect, the feedback voltage represents a secondary-side voltage of a secondary winding. Coupled to the comparator, the ON-triggering controller operates a power switch at about a first switching frequency when the feedback voltage is lower than the over-shot reference voltage. The ON-triggering controller operates the power switch at about a second switching frequency when the feedback voltage exceeds the over-shot reference voltage. The second switching frequency is less than the first switching frequency.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation application of U.S. patent application Ser. No. 13/650,098, filed on Oct. 11, 2012, and all benefits of such earlier application are hereby claimed for this new continuation application.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a primary side control (PSC) switching-mode power supply (SMPS), and particularly to a PSC SMPS that has reduced output voltage jitter.

2. Description of the Prior Art

Power supplies are a necessary electronic device in most electronic products, and are used for converting battery or grid power to power required by the electronic product and having specific characteristics. In most power supplies, switching-mode power supplies have superior electrical energy conversion efficiency and smaller product dimensions, making them popular in the power supply market.

Two different control schemes are used in current switching-mode power supplies: primary side control (PSC) and secondary side control (SSC). SSC directly couples a detection circuit to an output node of a secondary winding of a power supply, then through a photo coupler, transmits a detection result to a power supply controller located on the primary side to control energy of the power supply that is to be stored and converted on the primary winding. Compared to SSC, PSC indirectly detects voltage outputted by the secondary winding through directly detecting reflected voltage on an auxiliary winding, and indirectly completes detection of output voltage on an output node of the power supply. PSC completes detection and energy conversion control on the primary side. Compared to SSC, PSC is able to lower cost, as PSC does not require the photo coupler having both greater size and cost. PSC may also have higher conversion efficiency, because PSC does not require the detection circuit on the secondary side that constantly drains energy.

FIG. 1 is a diagram of a switching-mode power supply that uses PSC. Bridge rectifier 20 rectifies alternating current from grid node AC to establish direct current input power at input node IN. Voltage V_(IN) of output power may have an M-shaped waveform, but may also be filtered into a fixed level that roughly does not vary over time. Transformer has three windings: primary winding PRM, secondary winding SEC, and auxiliary winding AUX. Power supply controller 26 periodically controls power switch 34 through gate node GATE. When power switch 34 is ON, primary winding PRM performs energy storage. When power switch 34 is OFF, secondary winding SEC and auxiliary winding AUX discharge to establish output voltage VOUT on output node OUT for supply to load 24, and control voltage VCC for supply to power supply controller 26.

Voltage divider resistors 28, 30 detect voltage V_(AUX) of auxiliary winding AUX to provide feedback voltage V_(FB) to feedback node FB of power supply controller 26. According to feedback voltage V_(FB), power supply controller 26 establishes compensation voltage V_(COM) on compensation capacitor 32, and controls power switch 34 according thereto.

FIG. 2 shows the power supply controller 26 of FIG. 1 and some external components. Power supply controller 26 comprises sampler 12, pulse generator 14, transconductor 15, and pulse width controller 16. During discharging of secondary winding SEC and auxiliary winding AUX, pulse generator 14 provides a short pulse to sampler 12, so that sampler 12 samples feedback voltage V_(FB) to generate feedback voltage V_(IFB) at intermediate node IFB. Through feedback node FB, voltage divider resistors 28 and 30, and auxiliary winding AUX, feedback voltage V_(IFB) equivalently represents voltage level of secondary winding voltage V_(SEC) of secondary winding SEC during discharging, and roughly represents output voltage V_(OUT). Transconductor 15 controls compensation voltage V_(COM) on compensation node COMP according to a comparison result of feedback voltage V_(IFB) and target voltage V_(REF). Pulse width controller 16 controls power switch 34 according to compensation voltage V_(COM). Overall, power supply controller 26 provides a feedback mechanism that roughly stabilizes feedback voltage V_(IFB) to target voltage V_(REF), and is thus able to stabilize output voltage V_(OUT).

SUMMARY OF THE INVENTION

According to an embodiment, a primary-side control method comprises providing a feedback voltage, the feedback voltage representing a secondary-side voltage of a secondary winding through an inductance-coupling effect; controlling a power switch by a first switching frequency; comparing the feedback voltage and an over-shot reference voltage; and controlling the power switch by a second switching frequency when the feedback voltage is greater than the over-shot reference voltage. The second switching frequency is lower than the first switching frequency.

According to an embodiment, a power supply controller for performing primary-side control comprises a comparator and an ON triggering controller. The comparator is for comparing a feedback voltage and an over-shot reference voltage. The feedback voltage represents a secondary-side voltage of a secondary winding through an inductance-coupling effect. The ON-triggering controller is coupled to the comparator. When the feedback voltage is lower than the over-shot reference voltage, the ON-triggering controller causes a power switch to operate at approximately a first switching frequency. When the feedback voltage is higher than the over-shot reference voltage, the ON-triggering controller causes the power switch to operate at approximately a second switching frequency. The second switching frequency is lower than the first switching frequency.

According to an embodiment, a power management system comprises a transformer, a power switch, and a power supply controller. The transformer has a primary winding, an auxiliary winding, and a secondary winding. The power switch is coupled to the primary winding for controlling an inductance current flowing through the primary winding. The power supply controller is for controlling the power switch, and comprises a feedback node, a comparator, and an ON-triggering controller. The feedback node is coupled to the auxiliary winding. The comparator is for comparing a feedback voltage and an over-shot reference voltage. The feedback voltage represents a secondary-side voltage of the secondary winding through the feedback node and the auxiliary winding. The ON-triggering controller is coupled to the comparator. The ON-triggering controller causes the power switch to operate approximately at a first switching frequency when the feedback voltage is lower than the over-shot reference voltage, and the ON-triggering controller causes the power switch to operate approximately at a second switching frequency when the feedback voltage is higher than the over-shot reference voltage. The second switching frequency is lower than the first switching frequency.

These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a switching-mode power supply that uses PSC.

FIG. 2 shows the power supply controller of FIG. 1 and some external components.

FIG. 3 is a diagram of a power supply controller according to an embodiment.

FIG. 4 is a diagram of a power supply controller according to an embodiment.

DETAILED DESCRIPTION

In the following examples, components sharing the same reference numerals have similar or the same function, structure, and operation. Persons of ordinary skill in the art may arrive at simple alterations or modifications of the embodiments of the detailed description according to the teachings and disclosure herein without leaving the spirit of the present invention.

The power supply controller 26 of FIG. 2 may exhibit excessive output voltage VOUT jitter during light-heavy load switching.

For example, when load 24 suddenly transitions from a heavy load to a light load or no load, output voltage V_(OUT) will suddenly rise. And, power supply controller 26 must wait for a period of time, in which transconductor 15 pulls compensation voltage V_(COM) down to a certain level, such that energy converted by transformer is lower than energy consumed by load 24, before output voltage V_(OUT) can begin to fall. However, at this time, output voltage V_(OUT) is very likely to already have exceeded the required specification of the power supply management system.

FIG. 3 is a diagram of a power supply controller 26 _(a) according to an embodiment. Power supply controller 26 _(a) replaces power supply controller 26 of FIG. 1.

Power supply controller 26 _(a) comprises sampler 12, pulse generator 14, transconductor 15, comparator 60, oscillator 62, and pulse width controller 64.

After pulse width controller 64 turns power switch 34 off, secondary winding SEC and auxiliary winding AUX begin to release energy stored previously by primary winding PRM while power switch 34 was turned on. The time for secondary winding SEC and auxiliary winding AUX to release electrical energy is called discharge time T_(DIS). During discharge time T_(DIS), pulse generator 14 provides a short pulse to cause sampler 12 to sample feedback voltage V_(FB) on feedback node FB. A sample result is then stored on intermediate node IFB as feedback voltage V_(IFB). Thus, feedback voltage V_(IFB) approximately represents output voltage V_(OUT) through voltage division and inductive coupling through feedback node FB, voltage divider resistors 28 and 30, auxiliary winding AUX, and secondary winding SEC.

Transconductor 15 controls compensation voltage V_(COM) according to feedback voltage V_(IFB) and target voltage V_(REF). In some embodiments, pulse width controller 64 determines ON time T_(ON) of power switch 34 per one switching period according to compensation voltage V_(COM) on compensation node COMP, which is time in which power switch 34 is short circuited.

Oscillator 62 provides set signal S_(SET) through set node SET, which periodically triggers turning on of power switch 34. Thus, switching frequency of power switch 34 is approximately equal to frequency of set signal S_(SET). In some embodiments, frequency of set signal S_(SET) can be determined from compensation voltage V_(COM). For example, frequency of set signal S_(SET) can decrease with decreasing compensation voltage V_(COM).

Comparator 60 compares feedback voltage V_(IFB) and over-shot reference voltage V_(OS-REF). Comparison result S_(OV) of comparator 60 affects frequency of set signal S_(SET) provided by oscillator 62. For example, when feedback voltage V_(IFB) is lower than over-shot reference voltage V_(OS-REF), comparison result S_(OV) is logic 0, and frequency of set signal S_(SET) may be determined solely by compensation voltage V_(COM) to be, for example, 60 KHz. As soon as feedback voltage V_(IFB) exceeds over-shot reference voltage V_(OS-REF)/comparison result S_(OV) becomes logic 1, and frequency of set signal S_(SET) immediately drops to be fixed at, for example, 25 KHz.

Power supply controller 26 _(a) of FIG. 3 can suppress output voltage V_(OUT) jitter when transitioning from a heavy load to a light load. The following description is made with reference to FIG. 1, with power supply controller 26 _(a) replacing power supply controller 26 thereof, and target voltage V_(REF) and over-shot reference voltage V_(OS-REF) assumed to be 2.5V and 2.6V, respectively. As soon as load 24 suddenly transitions from heavy loading to light loading or no loading, because energy output of the transformer exceeds energy consumption of load 24, output voltage V_(OUT) suddenly rises, causing feedback voltage V_(IFB) to start rising in turn. As soon as feedback voltage V_(IFB) exceeds over-shot reference voltage V_(OS-REF) of 2.6V, frequency of set signal S_(SET) immediately drops to a low value, so that electrical power outputted by transformer immediately drops. Compared to the prior art, which must wait for compensation voltage V_(COM) to be pulled down to a certain level before transmitted energy can drop noticeably, as soon as power supply controller 26 _(a) discovers that feedback voltage V_(IFB) has exceeded over-shot reference voltage V_(OS-REF) of 2.6V, frequency of set signal S_(SET) is dropped immediately, which also lowers electrical power output of the transformer, thus rapidly prohibiting output voltage V_(OUT) from increasing.

Feedback voltage V_(IFB) is periodically updated as set signal S_(SET) periodically turns on power switch 34, so as to track current output voltage V_(OUT). As long as feedback voltage V_(IFB) is lower than over-shot reference voltage V_(OS-REF) of 2.6V, power supply controller 26 a will return to normal operation, e.g. frequency of set signal S_(SET) being determined only on by compensation voltage V_(COM). So, for normal operation, power supply controller 26 _(a) and power supply controller 26 are the same, each causing feedback voltage V_(IFB) to converge to target voltage V_(REF) of 2.5V.

FIG. 4 is a diagram of a power supply controller 26 _(b) according to an embodiment. In the following description, power supply controller 26 _(b) replaces power supply controller 26 of FIG. 1 as another embodiment.

Compared to the power supply controller 26 _(a) of FIG. 2, power supply controller 26 _(b) has OFF time controller 66 coupled to feedback node FB. OFF time controller 66 may employ valley switching. For example, after discharge time T_(DIS), auxiliary winding voltage V_(AUX) of auxiliary winding AUX starts oscillating, and gradually converges to 0V. So-called “valley switching” may mean that, after power switch 34 is turned off, power switch 34 is turned on when a 1^(st) valley, a 2^(nd) valley, a 3^(rd) valley, and so on of auxiliary winding voltage V_(AUX) occurs. This type of operating scheme is typically called quasi-resonance (QR) mode.

Through feedback node FB, OFF time controller 66 can determine when auxiliary winding voltage V_(AUX) drops across 0V, so-called zero crossing. OFF time controller 66 may be designed to trigger pulse width controller 64 to turn on power switch 34 through set node SET a predetermined period after auxiliary winding voltage V_(AUX) drops across 0V. Thus, valley switching can be approximately realized. In order to avoid zero-crossing never being detected, OFF time controller 66 can be designed to forcefully trigger pulse width controller 64 to turn on power switch 34 if no zero-crossing has been detected after a maximum OFF time.

In the embodiment of FIG. 4, when feedback voltage V_(IFB) is lower than over-shot reference voltage V_(OS-REF), comparison result S_(OV) is logic 0. At this time, timing of set signal S_(SET) triggering turning on of power switch 34 may be determined according to compensation voltage V_(COM) and zero-crossing detected by OFF time controller 66 through feedback node FB. Simply speaking, when feedback voltage V_(IFB) is lower than over-shot reference voltage V_(OS-REF), power supply controller 26 b approximately operates in QR mode, and may trigger turning on of power switch 34 at any valley appearing in auxiliary winding voltage V_(AUX).

When feedback voltage V_(IFB) is greater than over-shot reference voltage V_(OS-REF), comparison result S_(OV) is logic 1, and OFF time controller 66 only triggers pulse width controller 64 to turn on power switch 34 after maximum OFF time. At this time, switching frequency of power switch 34 is necessarily lower than when operating in QR mode.

Similar to power supply controller 26 _(a) of FIG. 3, when output voltage V_(OUT) is on the high side, causing feedback voltage V_(IFB) to exceed over-shot reference voltage V_(OS-REF), power supply controller 26 _(b) of FIG. 4 causes OFF time of power switch 34 to be maximum OFF time, so that switching frequency immediately drops. Electrical power transmitted by the transformer can be lowered rapidly, which can rapidly prevent output voltage V_(OUT) from rising further.

It is predictable that the power supply controllers of FIG. 3 and FIG. 4 can both rapidly prevent feedback voltage V_(IFB) from rising further, which can reduce output voltage V_(OUT) jitter, and cause output voltage V_(OUT) to converge more rapidly.

Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims. 

What is claimed is:
 1. A primary-side control method comprising: providing a feedback node, coupled to an auxiliary winding; generating a feedback voltage by sampling the feedback node during discharging of the secondary winding; controlling a power switch in a first switching frequency; comparing the feedback voltage and an over-shot reference voltage; and controlling the power switch in a second switching frequency when the feedback voltage is greater than the over-shot reference voltage; wherein the second switching frequency is lower than the first switching frequency, and the feedback voltage represents a secondary-side voltage of the secondary winding through an inductance-coupling effect.
 2. The primary-side control method of claim 1, further comprising: turning on the power switch if an auxiliary winding voltage of the auxiliary winding is approximately within a voltage valley, when the feedback voltage is lower than the over-shot reference voltage; and turning on the power switch after the power switch turned off for a maximum OFF time, when the feedback voltage is higher than the over-shot reference voltage.
 3. The primary-side control method of claim 1, further comprising: comparing the feedback voltage and a target voltage and a target voltage to control a compensation voltage; and controlling ON time of the power switch according to the compensation voltage.
 4. The primary-side control method of claim 1, further comprising: determining the first switching frequency according to the compensation voltage.
 5. A power supply controller for performing primary-side control, comprising: a comparator for comparing a feedback voltage and an over-shot reference voltage, wherein the feedback voltage represents a secondary-side voltage of a secondary winding through an inductance-coupling effect; a sampler coupled between the comparator and a feedback node coupled to an auxiliary winding, the sampler sampling the feedback node to generate the feedback voltage; and an ON-triggering controller coupled to the comparator, wherein when the feedback voltage is lower than the over-shot reference voltage, the ON-triggering controller causes a power switch to operate at approximately a first switching frequency, and when the feedback voltage is higher than the over-shot reference voltage, the ON-triggering controller causes the power switch to operate at approximately a second switching frequency; wherein the second switching frequency is lower than the first switching frequency.
 6. The power supply controller of claim 5, further comprising: a pulse generator for providing a pulse during discharging of the secondary winding for causing the sampler to sample the feedback node.
 7. The power supply controller of claim 6, further comprising: a transconductor for comparing the feedback voltage and a target voltage to control a compensation voltage.
 8. The power supply controller of claim 7, wherein the ON-triggering controller is an oscillator for providing a periodic signal to trigger turning on of the power switch, and the compensation voltage determines a switching frequency of the periodic signal.
 9. The power supply controller of claim 6, wherein: the ON-triggering controller is an OFF time controller coupled to a feedback node; the OFF time controller triggers turning on of the power switch if an auxiliary winding voltage of the auxiliary winding is approximately in a voltage valley, when the feedback voltage is lower than the over-shot reference voltage; and the OFF time controller triggers turning on of the power switch after the power switch turned off for a maximum OFF time, when the feedback voltage is larger than the over-shot reference voltage. 